The Third Generation Partnership Project (3GPP) unites six telecommunications standards bodies, known as “Organizational Partners,” and provides their members with a stable environment to produce the highly successful Reports and Specifications that define 3GPP technologies. These technologies are constantly evolving through what have become known as “generations” of commercial cellular/mobile systems. 3GPP also uses a system of parallel “releases” to provide developers with a stable platform for implementation and to allow for the addition of new features required by the market. Each release includes specific functionality and features that are specified in detail by the version of the 3GPP standards associated with that release.
Universal Mobile Telecommunication System (UMTS) is an umbrella term for the third generation (3G) radio technologies developed within 3GPP and initially standardized in Release 4 and Release 99, which preceded Release 4. UMTS includes specifications for both the UMTS Terrestrial Radio Access Network (UTRAN) as well as the Core Network. UTRAN includes the original Wideband CDMA (W-CDMA) radio access technology that uses paired or unpaired 5-MHz channels, initially within frequency bands near 2 GHz but subsequently expanded into other licensed frequency bands. The UTRAN generally includes node-Bs (NBs) and radio network controllers (RNCs). Similarly, GSM/EDGE is an umbrella term for the second-generation (2G) radio technologies initially developed within the European Telecommunication Standards Institute (ETSI) but now further developed and maintained by 3GPP. The GSM/EDGE Radio Access Network (GERAN) generally comprises base stations (BTSs) and base station controllers (BSCs).
Long Term Evolution (LTE) is another umbrella term for so-called fourth-generation (4G) radio access technologies developed within 3GPP and initially standardized in Releases 8 and 9, also known as Evolved UTRAN (E-UTRAN). As with UMTS and GSM/EDGE, LTE is targeted at various licensed frequency bands including the 700-MHz band in the United States. LTE is accompanied by improvements to non-radio aspects commonly referred to as System Architecture Evolution (SAE), which includes Evolved Packet Core (EPC) network. From the perspective of an end user, one of the most notable features of LTE is much higher data rates than those available in UTRAN or GERAN, which improves the user's experience in many applications including email, audio and video streaming, personal navigation, gaming, etc.
In practice, a plurality of networks using different radio-access technologies (e.g., a GERAN, a UTRAN, an E-UTRAN, and/or a CDMA2000 network) covering substantially the same geographic area may coexist on different frequencies under control of the same network operator. The operator may configure the respective networks such that a user equipment (UE) capable of communicating on any combination of radio-access technologies (i.e., “multi-RAT UE”) may transition between the available networks according to certain rules and requirements. As used within the 3GPP standards, “user equipment” or “UE” means any wireless communication device (e.g., smartphone or computing device) that is capable of communicating with GERAN, UTRAN, E-UTRAN, CDMA2000 network or other such network equipment.
Even if networks with multiple radio access technologies (RATs) are available to the multi-RAT UE, it is desirable for the multi-RAT UE to establish and maintain a connection to the E-UTRAN since it provide the user experience benefits described above. However, situations may arise where the UE cannot establish or maintain a connection with the E-UTRAN because it is overly congested. In such cases, the multi-RAT UE must look for service on networks using other RATs.